This invention relates to an internal combustion engine and, more particularly, to an internal combustion engine having a cylinder section which may be pivoted with respect to the crankcase section thereof to vary the compression ratio of the engine.
An in-line engine of the above type has a cylinder receiving section (a cylinder block) which is connected tiltably to the crankshaft supporting crankcase section of the engine by a hinge shaft bearing arrangement connecting the parts on one side of the engine. The cylinder receiving section supports a cylinder head which forms the cylinder head of the engine. On the other side of the engine there is a tilting mechanism arranged between the cylinder receiving section and the crankcase section, with which mechanism the cylinder receiving section and the cylinder head connected to it can be inclined laterally relative to the crankcase section.
Because the cylinder receiving section can be inclined (tilted) relative to the crankcase section, the distance between the crankshaft (with adhering pistons) and the cylinders will be variable. The volume of that part of the combustion chamber which is located above the upper limiting surface of the respective pistons, in the upper turning position of the piston (upper dead centre) can therefore be increased by the lateral inclination of the cylinder receiving section relative to the crankcase section. This means that the compression ratio of the engine will be variable, enabling the efficiency of the engine to be optimized for varying driving loads, which results in improved engine performance.
The tilting shaft bearing arrangement between the crankcase section and the cylinder receiving section is, as already mentioned, arranged on one side of the engine, while the tilting mechanism with which the cylinder receiving section can be inclined relative to the crankcase section is arranged on the opposite side of the engine. The tilting shaft bearing arrangement suitably incorporates a lateral inclination shaft which runs parallel with the crankshaft and which is housed in axially separated bearing brackets, which are rigidly connected to the crankshaft section and are positioned in line with each other along the outside of the cylinder receiving section, preferably in its lower region. In the intervals between the bearing brackets are situated bearing lugs in the cylinder receiving section mounted on the sections of the lateral inclination shaft located there. The tilting shaft bearing arrangement therefore consists of the bearing brackets, the lateral inclination shaft and the bearing lugs, which together form a type of longitudinal hinge mechanism between the crankcase section end the cylinder receiving section.
The tilting mechanism on the opposite side of the engine may, for example, incorporate essentially vertically directed rods resembling connecting rods, whose upper ends are swivelled on an upper bearing shaft parallel to the crankshaft and passing along the cylinder receiving section. The lower ends of the rods may then be eccentrically mounted on an eccentric shaft, which is in turn swivelled in bearing brackets rigidly connected to the crankcase section. The upper bearing shaft is in this case supported in the upper region of the cylinder receiving section by means of bearing brackets which are rigidly connected to the cylinder receiving section. The distance between the upper bearing shaft and the bearing brackets housing the eccentric shaft in the crankshaft section may therefore be varied by rotating the eccentric shaft. By varying this distance this side of the cylinder receiving section can be raised (or lowered) relative to the crankcase section, which gives rise to lateral inclination/tilting of the cylinder receiving section relative to the crankcase section.
The bearing brackets connected to the crankcase section for the lateral inclination shaft on one side of the engine, and the bearing brackets similarly connected to the crankcase section for the eccentric shaft, on the other side of the engine, are suitably positioned in pairs in planes between the cylinders perpendicular to the crankshaft, to increase stability. Consequently each pair of bearing brackets arranged opposite each other in the transverse direction of the engine may constitute integral parts of a common "transverse frame" of the crankcase section, which enables the crankcase to be dimensioned rationally, from the point of view of design and strength, to absorb the relatively large forces and moments applied to the crankcase section from the cylinder receiving section via the tilting shaft bearing arrangement and tilting mechanism respectively, particularly when combustion is taking place in the engine cylinders.
The transverse frame mentioned, or the bearing bracket sections of the crankcase section will, during combustion in the cylinders, be subjected to vertically upwardly directed tensile forces from the cylinder receiving section. These forces are then applied to the respective transverse frames in the form of upwardly directed forces concentrated on both the lateral inclination shaft bearing and on the eccentric shaft bearing in the frame.
Since both the lateral inclination shaft and the eccentric shaft are located laterally outside the cylinder receiving section, for design reasons, the upwardly directed forces applied to each such bearing bracket section (transverse frame) will attack the section at a considerable distance from the longitudinal centre plane of the engine in which the crankshaft is located. This means that the bearing bracket section has its maximum bending moment stress concentrated in one point in the aforementioned centre plane, which lies roughly in the centre of that part of the section/frame which is located immediately above the crankshaft. This part may then be regarded as a beam web bending moment loaded by upwardly directed end forces, i.e. the maximum compressive stresses caused by the bending moment at the upper limit (edge) of this web section occur in the aforementioned longitudinal centre plane, whilst the maximum tensile stresses occur at the lower limit of the web section, i.e. at the top of the inlet port for the crankcase in the frame. The above may be expressed by stating that the bearing brackets, because of their location a relatively long lateral distance from the centre plane of the engine, cause relatively large vertical forces to act on the crankcase section at a lateral distance from the centre plane of the engine, which results in considerable stresses in the crankcase section, and particularly in the region of the crankshaft bearings located at that point.
The above-mentioned bending moment caused by the upwardly directed forces, at the lateral inclination shah and eccentric shah respectively, also gives rise to a corresponding deflection of the lower lateral parts of the crankcase section, in the lateral direction, away from the longitudinal centre plane of the engine.
In order to reduce the maximum compressive and bending stresses in the transverse frame are above the crankshaft, and to limit the lateral deflection of the lower lateral parts of the crankcase section, it will therefore be necessary to make the crankcase section, and particularly its bearing bracket sections, much stronger, relatively speaking, than would be required in a convention, in-line engine design in which the cylinder block and the crankcase section are integrated to form a fixed unit, i.e. to form an engine block. In dimensioning such a conventional in-line engine there are of course no problems with upwardly directed forces generating bending moments at a tilting shaft bearing or tilting mechanism because the cylinder receiving section or the cylinder block is not in this case tiltably mounted on the crankshaft section but is rigidly connected to the same. On the other hand the conventional in-line engine does not provide the possibility of varying the engine compression, which is of course a particular objective of an internal combustion engine of the type indicated in the introduction.
As an example of prior art in this field it can be mentioned that U.S. Pat. No. 2,770, 224 describes and illustrates a multiple cylinder overhead valve engine in which a cylinder receiving section, with associated cylinder head/cover, is pivotably hinged to a stationary crankcase section. The cylinder receiving section of the engine can in this case be inclined (tilted) laterally relative to the crankcase section about a longitudinal guide shaft (lateral tilting shaft) on one longitudinal side of the engine.
This known engine is therefore of the type indicated in the introduction, in which the compression can be varied by inclining the cylinder receiving section relative to the crankcase section.
However, an obvious disadavantage of this known engine, with variable compression, is that the lower lateral parts of the crankcase section will be laterally deflected because of the bending moment to which the crankcase section is subjected as a result of the upwardly directed forces at the lateral tilting shaft and at the force transmitting parts of the tilting mechanism on the opposite side of the engine.